I am not sure what kind of object dsPopulateGrid is, but apparently it does not maintain its state during postbacks.
This means that when you post back (e.g.) click the Button, you'll loose the value of that variable.
You probably need to recreate the DataSource / object again after postback. You can do this by calling a custom method from the Page_Load event.

HtH,

Imar
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Imar Spaanjaars
Everyone is unique, except for me.
While typing this post, I was listening to: Pop Song For Us Rejects by Silverchair (Track 8 from the album: Freak Show)